1. Field of the Invention
The present invention relates to a gas-insulated switchgear that includes a sealed vessel in which components, such as a high-voltage conductor, a switch unit, and a current transformer, are housed, and insulates the components including the high-voltage conductor from the sealed vessel by filling the sealed vessel with insulating gas.
2. Description of the Related Art
A gas-insulated switchgear typically includes a cylindrical grounded tank and a cylindrical high-voltage conductor, which is coaxially arranged inside the tank, and contains sulfur hexafluoride as a main insulating medium. Because the dielectric strength of sulfur hexafluoride is approximately three times higher than that of air, use of sulfur hexafluoride as the insulating medium makes it possible to reduce a clearance between a high-voltage portion and an earthed electrode as compared with a conventional configuration, thereby configuring the switchgear compact.
However, when a foreign metallic particle enters into the tank, the foreign metallic particle can be electrically charged and radially reciprocate inside the tank, causing the withstand voltage of the equipment to decrease, so that it is necessary to remove such a foreign metallic particle or to control the motions of the foreign metallic particle. A foreign particle can enter the tank in particular through a sliding portion, because a current transformer is generally arranged near a circuit breaker and a disconnect switch.
The current transformer detects an electric current that is induced by a coil coaxially arranged with a high-voltage conductor. A conventional current transformer (see, for example, Japanese Examined Utility Model Application Publication No. H3-47323) includes a metal tube member, a high-voltage conductor that extends in the tube member, and a coil that is externally wound around the tube member. The tube member is arranged inside the sealed vessel. Because a distance between the high-voltage conductor and the tube member defines a gap distance for insulation, the sealed vessel has been required to be large in radial direction. It is necessary to reduce the gap distance to employ a smaller tank than a tank employed in the conventional current transformer; however, the strength of the electric field inside the tank increases as the electrical clearance decreases. Accordingly, increasing the insulating strength of the high-voltage conductor and the tube member, via which grounding is established, is necessary to use a smaller tank.
A disadvantage with the conventional configuration against the entry of a metal foreign material will be described. Because the strength of the electric field in a bottom of the tank other than a portion where the tube member is arranged is low, the foreign metallic particle is unlikely to be moved by the electric field. However, because the current transformer is generally arranged near the switch device, vibrations resulting from operation of the switch device can move the foreign metallic material into the tube member of the current transformer where the strength of the electric filed is higher than at the bottom of the tank. Hence, the foreign metallic particle entered the tube member can disadvantageously degrade insulating performance.
To this end, Japanese Examined Utility Model Application Publication No. H3-47323 discloses a technique for increasing the insulating strength by setting gentle curvatures for end portions of the tube member so as to reduce the strength of the electric field. This technique is based on a fact that the electric field has been likely to be dense at end portions of a tube member, and aims at suppressing electric discharge from the end portions by setting gentle curvatures for the end portions.
Japanese Laid-open Utility Model Application No. 562-57511 discloses a technique of arranging a corrugated foreign-particle trapping device on a bottom of a tank. By receiving a foreign metallic particle in a recess where the electric field is weak, the trapping device counteracts an adverse influence of the foreign metallic particle.
However, such a current transformer as disclosed in Japanese Examined Utility Model Application Publication No. H3-47323 is disadvantageous in that because the entire tube member serves as an earthing electrode, a ratio of a portion where the strength of electric field is low is small. When the ratio of the portion where the strength of the electric field is low is small, it is difficult to suppress motions of a foreign metallic particle, which can result in degradation in insulating performance.
The electric field in the current transformer is more uniform than that in a busbar. However, when a structure such as the corrugated foreign-particle trap device employed in Japanese Laid-open Utility Model Application No. S62-57511 is arranged on the inside bottom of the tube member, the strength of the electric field increases at peak portions of the corrugated shape, which can cause electric discharge from the peak portions to occur.